Method For Self-Aligning Solder-Attached Mems Die To A Mounting Surface

ABSTRACT

A method of attaching a MEMS die to a surface includes centering and rotationally aligning a solder perform on a solder surface of a body, centering and rotationally aligning a MEMS die on the solder preform, and heating the solder perform in a reflow process until the solder is molten and surface tension of the molten solder moves the MEMS die to a position where the surface tensions balance, and the MEMS die is centered on, and rotationally aligned with, the solder surface of the body.

BACKGROUND OF THE INVENTION

This invention relates in general to a Micro Electro Mechanical Systems(MEMS) die. In particular, this invention relates to an improved methodfor controlling solder flow and surface tension when attaching the MEMSdie to a surface such that the MEMS die will self-align to a desiredposition relative to the surface while the attachment solder is molten.

A MEMS die must typically be geometrically aligned with a mountingsurface for optimal wire bonding. Typically, MEMS die alignment is afunction of solder flow and solder surface tension. Non-uniform solderflow and insufficient solder surface tension however, is known to causeundesirable misalignment of solder-attached MEMS dies.

According to a known method, a round solder preform is placed onto around pedestal of a mounting body and a rectangular MEMS die is placedonto the round solder preform. Often, the MEMS die is aligned to adesired position on the pedestal by an assembler who visually positionsand hand-places the MEMS die without the aid of alignment tools.According to this known method, the surface tension of the molten soldermay be insufficient to maintain a desired rotational alignment of theMEMS die relative to the pedestal. Thus, it would be desirable toprovide an improved method for controlling solder flow and surfacetension during MEMS die attachment such that the MEMS die willself-align to a desired position relative to a surface of a mountingbody while the attachment solder is molten.

SUMMARY OF THE INVENTION

This invention relates to an improved method for controlling solder flowand surface tension when attaching the MEMS die to a surface of amounting body such that the MEMS die will self-align to a desiredposition relative to the surface while the attachment solder is molten.

In a first embodiment, a method of attaching a MEMS die to surfaceincludes centering and rotationally aligning a solder perform on asolder surface of a body, centering and rotationally aligning a MEMS dieon the solder preform, and heating the solder perform in a reflowprocess until the solder is molten and surface tension of the moltensolder moves the MEMS die to a position where the surface tensionsbalance, and the MEMS die is centered on, and rotationally aligned with,the solder surface of the body.

A second embodiment of the method of attaching a MEMS die to a surfaceincludes placing solder mask having a centrally formed flow area on asolder surface of a body, centering and rotationally aligning a solderperform on the solder surface of the body within the flow area of thesolder mask, centering and rotationally aligning a MEMS die on thesolder preform, and heating the solder perform in a reflow process untilthe solder is molten and surface tension of the molten solder moves theMEMS die to a position where the surface tensions balance and the MEMSdie is centered within the flow area of the solder mask.

A third embodiment of the method of attaching a MEMS die to a surfaceincludes forming a solder well in a center of a solder surface of abody, centering and rotationally aligning a solder perform within thesolder well, centering and rotationally aligning a MEMS die on thesolder preform, and heating the solder perform in a reflow process untilthe solder is molten and surface tension of the molten solder moves theMEMS die to a position where the surface tensions balance, and the MEMSdie is centered within the solder well.

Various aspects of this invention will become apparent to those skilledin the art from the following detailed description of the preferredembodiments, when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a portion of a Superheat Controller (SHC) thathas been assembled according to a first embodiment of the method of thisinvention.

FIG. 2 is an enlarged cross-sectional elevational view taken along theline 2-2 of FIG. 1.

FIG. 3 is a perspective view of a portion of a known SHC.

FIG. 4 is a plan view of the portion of the known SHC illustrated inFIG. 3.

FIG. 5 is a perspective view of a known SHC.

FIG. 6 is a cross sectional view of the known SHC illustrated in FIG. 5.

FIG. 7 is a plan view of a portion of an SHC that has been assembledaccording to a second embodiment of the method of this invention.

FIG. 8 is an enlarged cross sectional elevational view taken along theline 8-8 of FIG. 7.

FIG. 9 is a plan view of a portion of an SHC that has been assembledaccording to a third embodiment of the method of this invention.

FIG. 10 is an enlarged cross sectional elevational view taken along theline 10-10 of FIG. 9.

FIG. 11 is an enlarged cross-sectional elevational view of the portionof the SHC illustrated in FIGS. 9 and 10 showing the solder solidifiedafter the perform has been heated and subsequently cooled.

FIG. 12 is a perspective view of the portion of the SHC illustrated inFIGS. 1 and 2 showing the PCB attached thereto.

FIG. 13 is a flow chart illustrating the first embodiment of the methodof this invention.

FIG. 14 is a flow chart illustrating the second embodiment of the methodof this invention.

FIG. 15 is a flow chart illustrating the third embodiment of the methodof this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, there is illustrated in FIGS. 1 and 2 aportion of a superheat controller (SHC) 52 that has been assembledaccording to a first embodiment of the method of this invention,described below. The SHC portion 52 includes a fluid inlet member 58having a pedestal 60. Advantageously, the pedestal 60 has asubstantially rectangular peripheral edge 63 (see FIG. 1).

As shown in FIGS. 1 and 2, the pressure sensor die 46 and its attachedglass cover 50 are bonded to an outwardly facing solder surface 61 ofthe pedestal 60 (the upwardly facing surface when viewing FIG. 2) by asolder perform 54 having a substantially rectangular peripheral edge 55and a hole 57 formed centrally therethrough.

Referring now to FIGS. 3 and 4, one embodiment of a portion 5 of a knownSHC (not shown, but substantially similar to the SHC 10 shown in FIGS. 5and 6) that has been assembled in a known manner is illustrated. Theportion 5 of the SHC shown in FIGS. 3 and 4 includes a body or basedefining a fluid inlet member 40. The fluid inlet member 40 is similarto the fluid inlet member 18 described below and includes asubstantially cylindrical pedestal 42 formed on a first end of the fluidinlet member 40. The fluid inlet member 40 includes a central portion 44that may include external threads. The illustrated fluid inlet member 40is formed from brass. Alternatively, the fluid inlet member 40 may beformed form other metals, metal alloys, and non-metal materials.

U.S. Pat. No. 9,140,613 discloses a superheat controller (SHC). The SHCdisclosed therein is a single, self-contained, stand-alone device whichcontains all the sensors, electronics, and intelligence to automaticallydetect a fluid type, such as refrigerant, and report the superheat ofmultiple common fluid types used in residential, industrial, andscientific applications. U.S. Pat. No. 9,140,613 is incorporated hereinin its entirety.

FIGS. 5 and 6 herein illustrate a SHC 10, which is similar to thesuperheat controller disclosed in U.S. Pat. No. 9,140,613. Theembodiment of the SHC 10 illustrated in FIGS. 5 and 6 includes a housing12 having a body 14, a cover 16, and the base that defines a fluid inletmember 18. The fluid inlet member 18 may be secured to the housing 12 bya mounting ring 19. The mounting ring 19 attaches the fluid inlet member18 to the housing 12 portion by a threaded connection. Alternatively,the mounting ring 19 may be attached to the fluid inlet member 18 by anydesired method, such as by welding or press fitting. In the embodimentillustrated in FIGS. 5 and 6, the fluid inlet member 18 is a brassfitting having a centrally formed opening that defines a sealing surface20.

In a known manner, a bore 41 (not shown in FIG. 3, but shown in FIG. 8)formed in the fluid inlet member 40 may convey pressurized fluid to bemeasured through the fluid inlet member 40, through a hermetic sealdefined by the solder perform 48 (when hardened after solder reflow),and into a pressure sensing chamber 46A (see FIG. 2) of a MEMS dieconfigured as pressure sensor die 46, described below. As shown in FIG.8, a portion 41A of the bore 41 that extends through the pedestal 42 mayhave a diameter smaller than a diameter of a portion 41B of the bore 41that extends through the rest of the fluid inlet member 40.

The SHC 10 includes an integrated pressure and temperature sensor 22having pressure sensor portion 24 and a temperature sensor portion 26mounted to a printed circuit board (PCB) 28. The superheat processor 30,a data-reporting or communication module 32, and an Input/Output (TO)module 34 are also mounted to the PCB 28. The IO module 34 is a physicalhardware interface that accepts input power and reports data throughavailable hard-wired interfaces, such as wires or cables 36, to thesuperheat processor 30. Target devices 38 that may be connected to theSHC 10 via the IO module 34 may include additional temperature sensors,laptop and notebook computers, cell phones, memory cards, and any deviceused in or with conventional end of the line test equipment.Alternatively, the target devices 38 may be connected to thecommunication module 32 by a wireless connection.

The superheat processor 30 is mounted to the PCB 28 and is ahigh-resolution, high accuracy device that processes the input signalsfrom the pressure and temperature sensor portions 24 and 26,respectively, of the integrated pressure and temperature sensor 22,detects the fluid type, calculates the superheat of the fluid, andprovides an output that identifies the level of the calculatedsuperheat. The superheat processor 30 may also be configured to provideother data, such as fluid temperature, fluid pressure, fluid type,relevant historical dates maintained in an onboard memory (such as alarmand on-off history), and other desired information. Advantageously, thesuperheat processor 30 maintains a high level of accuracy over a typicaloperating range of pressure and temperature after a one-timecalibration. Non-limiting examples of suitable superheat processorsinclude microcontrollers, Field Programmable Gate Arrays (FPGAs), andApplication Specific Integrated Circuits (ASICs) with embedded and/oroff-board memory and peripherals.

As shown in FIGS. 3 and 4, the pressure sensor die 46 is attached to thepedestal 42 of the fluid inlet member 40 by a solder perform 48. Thepressure sensor die 46 includes a glass cover 50 bonded to an outwardlyfacing surface thereof (the upwardly facing surface when viewing FIG.3). The pressure sensor die 46 and its attached glass cover 50 arefurther bonded to an outwardly facing solder surface 43 of the pedestal42 (the upwardly facing surface when viewing FIG. 3). A lower surface ofthe pressure sensor die 46 defines a bonding surface 49 (the downwardlyfacing surface when viewing FIG. 2). The known solder perform 48 has asubstantially circular shape, and the pressure sensor die 46 may bealigned thereon by an assembler by visual positioning and handplacement, i.e., without the aid of alignment tools. Thus, according tothis known method, the solder tension between the solder preform 48(when in a molten state) and the pressure sensor die 46 is insufficientto consistently center and rotationally align the pressure sensor die 46to a desired position relative to the pedestal 42.

Referring again to FIGS. 1 and 2, the substantially rectangular shape ofthe solder perform 54 advantageously guides and controls solder flow andsurface tension. Additionally, the application of solder via the solderperform 54 as shown in FIGS. 1 and 2 may significantly reduce or preventheated solder from flowing into surface ports or holes (not shown) inthe pressure sensor die 46.

As shown in FIG. 2, the glass cover 50 defines a cavity 50 a therein,and may be bonded to the outwardly facing surface of the pressure sensordie 46 (the upwardly facing surface when viewing FIG. 2) by any knownbonding method. The glass cover 50 may be positioned over a pressuresensing chamber 46A of the pressure sensor die 46. Once bonded to thepressure sensor die 46, the cavity 50 a of the glass cover 50 may beevacuated to a high vacuum so that the pressure sensor die 46 will readabsolute pressure rather than a gauge pressure, i.e., the pressuredifference from surrounding atmosphere. The vacuum formed in the cavity50 a may be achieved using known methods.

In the SHC portion 52 shown in FIGS. 1 and 2, the pressure sensor die 46may be packaged to mount on a base, i.e., the fluid inlet member 40,that has at one end (see the lower end of FIG. 3) a shape similar to thecore of a known Schrader valve (not shown). Therefore, a Schrader valvebody (not shown) may be easily connected to a system to be monitored,such as a conventional heating, ventilating, air conditioning, andrefrigeration (HVAC-R) system, but a core of the Schrader valve (notshown) may be replaced with the fluid inlet member 40 that will screwinto the Schrader Valve body (not shown) in the same way the core of theSchrader Valve does. In a known manner, a bore 56 in the fluid inletmember 58 may convey pressurized fluid to be measured through the fluidinlet member 58, through a hermetic seal defined by the solder perform54, and into a pressure sensing chamber 46A of the pressure sensor die46.

The first embodiment of the method of this invention is shown at 92 inFIG. 13 and may be performed to assemble the pressure sensor die 46 tothe pedestal 60, as shown in FIGS. 1 and 2. For example, the pedestal 60may be formed having the substantially rectangular peripheral edge 63and the pressure sensor die 46 may be formed having the substantiallyrectangular peripheral edge 47. Thus, the shape of the pedestal 60 andthe shape of the solder preform 54 are the same as the shape of aperipheral edge 47 of the pressure sensor die 46. An assembler may theneasily center and rotationally align the substantially rectangularsolder preform 54 on the substantially rectangular pedestal 60, andcenter and rotationally align the substantially rectangular pressuresensor die 46 on the substantially rectangular solder preform 54. Thesubstantially rectangular pressure sensor die 46 may therefore bepositioned in only one of two positions on the substantially rectangularsolder preform 54.

Indicia may be added to the pressure sensor die 46 to indicate which ofthe two possible positions is correct. Additionally, the pressure sensordie 46, the solder preform 54, and the pedestal 60 may have any othermatching shapes, so as to allow a constant width margin between theperipheral edges 47, 55, and 63, respectively, such as a square, anisosceles trapezoid, a triangle, and other desired polygonal shapes.

The fluid inlet member 58, the pressure sensor die 46, and the solderpreform 54 therebetween may be heated to initiate solder reflow, thusallowing the pressure sensor die 46 to self-align relative to thepedestal 60 while the solder preform 54 is molten. The solder preform 54may then be allowed to cool and harden.

Preferably, the peripheral edge 63 of the surface of the pedestal 60;i.e., the outer boundary of the solder-receiving surface, will be largerthan a surface of the pressure sensor die 46 by a ratio determined byroutine experimentation. This ratio may be optimized in a way thatensures balanced distribution and surface tension of molten solderpreform 54 during reflow. In this manner, the surface tension of themolten solder will act to move the pressure sensor die 46 to a positionwhere the surface tensions balance. This balance will occur when thepressure sensor die 46 is centered on, and rotationally aligned with,the shape of the pedestal 60 such that a constant width margin existsbetween the peripheral edges 47 and 63 of the pressure sensor die 46 andthe pedestal 60, respectively. The fluid inlet member 58, the pressuresensor die 46, and the solder preform 54 therebetween may then be heatedin a conventional manner to initiate solder reflow, thus allowing thepressure sensor die 46 to self-align relative to the pedestal 60 of thefluid inlet member 58 while the solder preform 54 is in a molten state.The heated fluid inlet member 58 and the solder preform 54 thereon maythen be allowed to cool and harden.

Thus, this method controls the flow of the molten solder preform 54 andsurface tension thereof during pressure sensor die 46 attachment in sucha manner that an amount of self-alignment of the pressure sensor die 46to a desired rotational position relative to the pedestal 60 will occurwhile the attachment solder perform 54 is molten.

It has been shown that improved bonding occurs if the solder preform 54shape matches the shape of the pressure sensor die 46. Bonding betweenthe pressure sensor die 46 and the pedestal 60 may be further improvedwhen the surface areas of the solder preform 54 and the pressure sensordie 46 are equal or substantially equal. For example, the use of thesubstantially rectangular solder preform 54 on the substantiallyrectangular pedestal 60, and the substantially rectangular pressuresensor die 46 on the substantially rectangular solder preform 54 mayminimize the distance that molten solder will have to flow until soldersurface tension centers and rotationally aligns the pressure sensor die46 relative to the pedestal 60. However, it will be understood that anon-rectangular solder preform 54 may work just as well with a similarlyshaped non-rectangular pedestal 60.

Referring now to FIGS. 7 and 8, there is illustrated at 62 a portion ofan SHC that has been assembled according to a second embodiment of themethod of this invention (see FIG. 14). The SHC portion 62 includes thefluid inlet member 40 having the substantially cylindrical pedestal 42,i.e., having a substantially circular cross-sectional shape. As shown inFIGS. 7 and 8, the pressure sensor die 46 and its attached glass cover50 (not shown in FIG. 7) are bonded to the solder surface 43 of thepedestal 42 by the solder perform 54 having the substantiallyrectangular peripheral edge 55 and the hole 57 formed centrallytherethrough.

In the second embodiment of the method, a solder mask 64 may be used toguide and control solder flow and surface tension. The illustratedsolder mask 64 (also known as solder resist) may be a thin coating of amaterial to which liquid solder will not bond. The solder mask 64 has asubstantially circular peripheral edge 65 and a diameter substantiallyequal to an outside diameter of the pedestal 42. A substantiallyrectangular flow area 66 may be formed in a center of the solder mask64. As described above, the solder perform 54 has the substantiallyrectangular peripheral edge 55 and may be positioned in the flow area 66between the pedestal 42 and the pressure sensor die 46.

By placing the solder mask 64 on the pedestal 42 in an area where solderis not desired, the flow area 66 defines an area in which molten solderfrom the solder preform 54 will be confined and where solder is desiredto bond with the pedestal 42. Within the flow area 66, the flow andsurface tension of the molten solder will act to center and rotationallyalign the pressure sensor die 46 to a desired orientation relative tothe flow area 66, and thus relative to the pedestal 42.

The second embodiment of the method of this invention is shown at 94 inFIG. 14. In a first step of the second embodiment of the method 94, athin layer of the solder mask 64 may be applied to a surface of thepedestal 42. The solder mask 64 may have any desired thickness, such aswithin about 5 μm to about 15 μm. The shape of the flow area 66 of thesolder mask 64 mask may be formed or selected to be substantially thesame as the shape of the peripheral edge 44 of the pressure sensor die46. The solder mask 64 may be positioned in a desired location androtational alignment relative to the pedestal 42.

Preferably, the flow area 66 of the solder mask 64 will be larger than asurface of the pressure sensor die 46, and a surface of the solderpreform 54, by a predetermined ratio, wherein the surface of thepressure sensor die 46 may be defined by the peripheral edge 47 thereof.This ratio of the size and depth of the flow area 66 to the peripheraledge 55 and thickness of the solder preform 54, and the size of theportion 41A of the bore 41 in the pedestal 42 may be optimized in a waythat ensures balanced distribution and surface tension of the moltensolder preform 54 during reflow. In this manner, the surface tension ofthe molten solder will act to move the pressure sensor die 46 to aposition where the surface tensions balance. This balance will occurwhen the pressure sensor die 46 is centered on, and rotationally alignedwith, the shape of the flow area 66 of the solder mask 64 such that aconstant width margin exists between the peripheral edge of the pressuresensor die 46 and the peripheral edge of the flow area 66. Thus, thisarrangement controls solder flow and surface tension during pressuresensor die 46 attachment in such a manner that an amount of pressuresensor die 46 self-alignment to a desired position relative to thepedestal 42 will occur while the attachment solder perform 54 is molten.

Preferably, the shape of the solder preform 54 should match the shape ofthe flow area 66 (but not necessarily the dimensions of the flow area66). For example, in addition to the embodiment shown in FIGS. 7 and 8,a non-rectangular solder preform (not shown) may perform just as wellwithin a properly optimized flow area 66 in the solder mask 64.

Referring now to FIGS. 9 through 11, there is illustrated at 72 aportion of an SHC that has been assembled according to a thirdembodiment of the method of this invention (see FIG. 15). The SHCportion 72 includes a fluid inlet member 74 having a substantiallycylindrical pedestal 76. As shown in FIGS. 9 through 11, the pressuresensor die 46 and its attached glass cover 50 (not shown in FIGS. 9through 11) are bonded to an outwardly facing surface of the pedestal 76(upwardly facing surface when viewing FIGS. 10 and 11) by a solderperform 78 having a substantially rectangular peripheral edge and acenter-hole 79 (shown in FIGS. 10 and 11) formed centrally therethrough.

In a known manner, a bore 75 formed in the fluid inlet member 74 mayconvey pressurized fluid to be measured through the fluid inlet member74, through a hermetic seal defined by the solder perform 78, and intothe pressure sensing chamber 46A of the pressure sensor die 46. As shownin FIGS. 10 and 11, a portion 75A of the bore 75 that extends throughthe pedestal 76 may have a diameter smaller than a diameter of a portion75B of the bore 75 that extends through the rest of the fluid inletmember 74.

A shallow solder well 80 is formed centrally in the pedestal 76. Theportion of the pedestal surrounding the solder well 80 defines acircumferentially and outwardly extending wall 77 (upwardly extendingwhen viewing FIGS. 10 and 11). The solder well 80 is configured to guideand control solder flow and surface tension. The illustrated solder well80 has a substantially rectangular shape. Alternatively, the solder well80 may have any desired shape, such as a shape corresponding to theshape of the peripheral edge of the die used, such as for example thepressure sensor die 46. The solder well 80 may have any desired depth,such as within about 0.1 mm to about 0.35 mm. It will be understood thatthe depth of the solder well 80 may vary based on a height of thepressure sensor die 46, the preform volume, and the structure of thebonding surface 49 thereof.

The third embodiment of the method of this invention is shown at 96 inFIG. 15. In a first step of the third embodiment of the method 96, theshallow solder well 80 may be formed centrally in the pedestal 76 andpreferably has a shape corresponding to a shape of a peripheral edge ofthe pressure sensor die 46. The solder well 80 may be formed such thatit is positioned in a desired location and rotational alignment relativeto the pedestal 76. The solder perform 78 may be positioned on a surfaceof the solder well 80.

Preferably, the solder well 80 will be larger than the bonding surface49 of the pressure sensor die 46 by a predetermined ratio, wherein thesurface of the pressure sensor die 46 may be defined by the peripheraledge thereof. This ratio of the size and depth of the solder well 80 tothe peripheral edge, thickness, and center-hole size of the solderpreform 78 may be optimized in a way that ensures balanced distributionand surface tension of molten solder during reflow. In this manner, thesurface tension of the molten solder will act between an outer wall ofthe solder well 80 and the peripheral edge of the pressure sensor die 46to move the pressure sensor die 46 to a position where the surfacetensions balance. This balance will occur when the pressure sensor die46 is centered in, and rotationally aligned with, the shape of thesolder well 80 such that a constant width margin exists between theperipheral edge of the pressure sensor die 46 and the peripheral edge;i.e., the outer walls, of the solder well 80. Thus, this arrangementcontrols solder flow and surface tension during pressure sensor die 46attachment in such a manner that an amount of pressure sensor die 46self-alignment to a desired position relative to the pedestal 76 willoccur while the attachment solder 78 is molten.

Preferably, the shape of the solder preform 78 should match the shape ofthe pressure sensor die 46 (but not necessarily the peripheral edgedimensions of the pressure sensor die 46). For example, in addition tothe embodiment shown in FIG. 9, a non-rectangular solder preform (notshown) may perform just as well with a properly optimized solder well80.

It will be understood that during assembly of the SHC portion 72, moltensolder 78 may flow into the bore 75 from the solder well 80. There maybe several ways to prevent this undesirable flow of solder 78. First,the pressure sensing chamber 46A in the pressure sensor die 46 may belarger in size than a diameter of the bore 75 formed through thepedestal 76. It has been shown that when the solder preform 78 has anannular shape and melts in the solder well 80, careful distribution of asolder flux (not shown) applied to the bonding surface on the bottom ofthe pressure sensor die 46 (but not in the pressure sensing chamber 46Ain the bottom of the pressure sensor die 46), will cause molten solder78 to be preferentially attracted to the bonding surface on the bottomof the pressure sensor die 46. Surface tension of the solder 78 willthen keep the molten solder 78 from flowing into the area under thepressure sensing chamber 46A in the pressure sensor die 46, such thatmolten solder 78 will not reach the bore 75.

Alternatively, careful control of the shape and size of the solderpreform 78 may be adequate to prevent molten solder from reaching thebore 75. For example, by providing a very thin solder preform 78 thatextends from the walls of the solder well 80 to a position underneathsome or all of the bonding surface on the bottom of the pressure sensordie 46, but not further radially inwardly, such as beneath the pressuresensing chamber 46A, advantageously results in insufficient soldervolume for significant portions of the solder to reach the bore 75 afterthe solder melts.

Additionally, a raised lip 90 may be formed circumferentially around thebore 75, over which the molten solder will not flow, thus preventing thesolder from flowing into the bore 75 (see FIGS. 10 and 11). As shown inFIGS. 10 and 11, the raised lip 90 may be formed such that it extendsoutwardly (upwardly when viewing FIGS. 10 and 11) from the pedestal 76and into the pressure sensing chamber 46A in the pressure sensor die 46to prevent solder from flowing into the bore 75. It will be understoodthat the lip 90 is not required.

After mounting the pressure sensor die 46 to any of the fluid inletmembers 40, 58, and 74, a PCB, such as the PCB 28 may then be mounted tothe fluid inlet member 40, 58, and 74, as shown in FIGS. 6 and 12. Theillustrated PCB 28 includes a die aperture 82 and two fastener apertures84. The PCB 28 may be positioned on the fluid inlet member 40, 58, and74 such that the pressure sensor die 46 extends through the die aperture82 and may be attached to the fluid inlet members 40, 58, and 74 withfasteners, such as threaded fasteners 86 (see FIG. 6) that extendthrough the fastener apertures 84 and into threaded bores 88 formed inthe fluid inlet members 40, 58, and 74.

The principle and mode of operation of this invention have beenexplained and illustrated in its preferred embodiment. However, it mustbe understood that this invention may be practiced otherwise than asspecifically explained and illustrated without departing from its spiritor scope.

What is claimed is:
 1. A method of attaching a MEMS die to a surface,the method comprising: centering and rotationally aligning a solderperform on a solder surface of a body; centering and rotationallyaligning a MEMS die on the solder preform; and heating the solderperform in a reflow process until the solder is molten and surfacetension of the molten solder moves the MEMS die to a position where thesurface tensions balance, and the MEMS die is centered on, androtationally aligned with, the solder surface of the body.
 2. The methodof attaching a MEMS die to a surface according to claim 1, wherein theMEMS die has a polygonal peripheral edge, the solder perform has apolygonal peripheral edge, and the portion of the body defining thesolder surface has a polygonal peripheral edge, and wherein each of theMEMS die, the solder perform, and the portion of the body defining thesolder surface have the same polygonal shape.
 3. The method of attachinga MEMS die to a surface according to claim 2, wherein the portion of thebody defining the solder surface is larger than the solder perform andthe MEMS die.
 4. The method of attaching a MEMS die to a surfaceaccording to claim 2, wherein the polygonal peripheral edge defines arectangle.
 5. The method of attaching a MEMS die to a surface accordingto claim 1, further including positioning the solder perform on thesolder surface of the body such that their respective peripheral edgesare aligned.
 6. The method of attaching a MEMS die to a surfaceaccording to claim 5, further including positioning the MEMS die on thesolder perform such that their respective peripheral edges are aligned.7. The method of attaching a MEMS die to a surface according to claim 6,wherein during the heating step the MEMS die is aligned with the soldersurface of the body such that a constant width margin exists between theperipheral edges of the MEMS die and the peripheral edges of the soldersurface of the body.
 8. A method of attaching a MEMS die to a surface,the method comprising: placing solder mask having a centrally formedflow area on a solder surface of a body; centering and rotationallyaligning a solder perform on the solder surface of the body within theflow area of the solder mask; centering and rotationally aligning a MEMSdie on the solder preform; and heating the solder perform in a reflowprocess until the solder is molten and surface tension of the moltensolder moves the MEMS die to a position where the surface tensionsbalance and the MEMS die is centered within the flow area of the soldermask.
 9. The method of attaching a MEMS die to a surface according toclaim 8, wherein the solder surface of the body and the solder preformhave dissimilar shapes.
 10. The method of attaching a MEMS die to asurface according to claim 9, wherein the solder surface of the body issubstantially circular and the solder preform is substantiallyrectangular.
 11. The method of attaching a MEMS die to a surfaceaccording to claim 10, wherein the centrally formed flow area of thesolder mask is substantially rectangular.
 12. The method of attaching aMEMS die to a surface according to claim 10, wherein the solder performhas a hole formed centrally therethrough.
 13. The method of attaching aMEMS die to a surface according to claim 12, further includingpositioning the solder perform within the flow area of the solder masksuch that the peripheral edges of the solder preform are rotationallyaligned with walls of the flow area of the solder mask.
 14. The methodof attaching a MEMS die to a surface according to claim 5, whereinduring the heating step the MEMS die is aligned with the solder surfaceof the body such that a constant width margin exists between theperipheral edges of the MEMS die and the peripheral edges of the soldersurface of the body.
 15. A method of attaching a MEMS die to a surface,the method comprising: forming a solder well in a center of a soldersurface of a body; centering and rotationally aligning a solder performwithin the solder well; centering and rotationally aligning a MEMS dieon the solder preform; and heating the solder perform in a reflowprocess until the solder is molten and surface tension of the moltensolder moves the MEMS die to a position where the surface tensionsbalance, and the MEMS die is centered within the solder well.
 16. Themethod of attaching a MEMS die to a surface according to claim 15,wherein the solder surface of the body and the solder preform havedissimilar shapes.
 17. The method of attaching a MEMS die to a surfaceaccording to claim 16, wherein the solder surface of the body issubstantially circular and the solder preform is substantiallyrectangular.
 18. The method of attaching a MEMS die to a surfaceaccording to claim 15, wherein the solder well is substantiallyrectangular.
 19. The method of attaching a MEMS die to a surfaceaccording to claim 10, wherein the solder perform has a hole formedcentrally therethrough.
 20. The method of attaching a MEMS die to asurface according to claim 15, further including a lip formed within thesolder well and extending circumferentially around a bore in the body,the lip defining a barrier over which molten solder will not flow duringthe heating step, the lip thus preventing the molten solder from flowinginto the bore.